Analysis of stress on threaded hole in square rod

[blkfrd]

Sorry I dont have any pics, but i'll do my best to describe this.

I have a 1" deep 3/4-16 threaded hole drilled down the center axis of a the end of a 1.25" square steel rod. A bolt goes thru the center of a high strength rod end (heim joint) and the bolt threads into the 3/4-16 hole the full 1" deep. The rod end has a pull force on it that imposes a shear force on the bolt which imparts the force onto the threaded hole perpendicular to the hole axis. I'm not concerned about the shear strength of the bolt. The weak link is the square rod that the bolt is screwed into. If you take a cross section of the 1.25" square rod along the length of the 3/4" threaded hole, the area of the square rod on either side of the hole along its length is .25" x 1" or 0.25 sq inches. Adding up the material on both sides of the hole is 0.5 sq inches total.

If the square rod has yield strength of 36,000 lbs, one may think the threaded hole will begin to deform when 18,000 lbs of shear force is applied to the bolt (0.5 sq inches x 36,000 lbs). An ME at work told me that a better approximation of when the threaded hole would begin to deform would be to consider only 1/2 the hole depth. The force applied against the hole by the bolt shear stress is in one direction at the top of the hole and in the opposite direction at the bottom of the hole. So according to the ME, a ballpark estimation is that the threaded hole would begin to deform for 9,000 lbs of shear force on the bolt.

Is this a good estimation? I'm not looking for exact figures at this time.

Tracy Blackford
Comm and GPS Systems Engineer

 

[BAretired]

I don't think it is a rational approach, so it is unlikely to be a good estimate. A FEM analysis could be used. It seems to me there would be a high splitting force at the end of the square bar. Not sure how to analyze it offhand.

BA

 

[bridgebuster]

Maybe I'm not visualizing this correctly, but, could you assume that half of the cross-section of the square rod is a cantilever beam over the length of the hole? The pull force would be at the end of the "cantilever beam".

 

[BAretired]

Maybe a sketch would help, but for a 3/4" bolt, the minimum edge distance from the center of bolt to an edge should be 25 mm (1.0") according to CSA S16. The actual edge distance is 5/8" (16 mm).

If a bolt shear is applied outside the square rod, a moment is applied to the bolt causing a variable compression on the edge of the hole with maximum value at the surface. The circular shape of the bolt would act like a wedge, causing a splitting force on the thin 1/4" wall. The centroid of the splitting force would be at least 1/3" inside the top of the rod and would move down as the material started to yield. This in turn would cause the force to increase making matters worse.

It is doubtful that the assembly would be able to safely sustain a 9,000# shear force.



BA